1. Field of the Invention
The present invention relates to a method and apparatus for correcting the moire effect on a multi-scan display monitor.
2. Background Information
Color cathode ray tubes (CRTs) are commonly used as visual display devices, employing up to three electrodes, typically one for each primary color: red, green, and blue. Most color CRTs use a shadow mask to selectively illuminate a matrix of each electrode's respective colored phosphors (i.e., red, green, and blue). The beam is typically larger than the shadow mask opening size, so the shadow mask blocks part of the beam and casts a portion of the original beam onto the desired phosphor. The dot pitch, or spacing, between adjacent shadow mask openings is typically between 0.2 and 0.3 mm to achieve a high resolution display.
A video pattern displayed on the CRT is comprised of a array of pixels, typically 640 to 1280 pixels wide and 480 to 1024 pixels high. One pixel will typically illuminate several screen dots. The number of screen dots illuminated is inversely proportional to the resolution of the video pattern. At higher video resolutions the number of screen dots illuminated can be less than three. At these higher resolutions an interference pattern, typically termed the moire effect, can appear. From an operating standpoint, the moire interference phenomenon poses a serious aesthetic problem, since the best electron beam focus and the highest image resolution results in generating an unacceptably noticeable moire effect if the video signal being displayed includes alternating pixel patterns, which is a common occurrence.
One method of correcting the moire effect is to rapidly shift the position of the image a small distance. The moire patterns rapidly change in response to the shifting position and, because of persistence of vision, become unnoticeable. The required shift in position is small, typically about one-half of the screen dot pitch, and is imperceptible when the screen is viewed. One method of position shifting moire cancellation is to shift all the lines of the display horizontally in every other display frame. Another method of position shifting moire cancellation is to shift every other line of the display horizontally.
The disclosed methods add additional circuitry to the display monitor to produce the shifting of the image, increasing the cost and complexity of the display monitor. What is needed is a method and apparatus for correcting the moire effect that reduces or eliminates the need for additional circuitry in the display monitor and which is suitable for use in display monitors which operate with a plurality of scanning resolutions and frequencies.